Apparatus for imparting high energy to charged particles



4 Sheets-Sheet 1 A. M. GUREWITSCH APPARATUS FOR IMPARTING HIGH ENERGY TO CHARGED PARTICLES .vlllluluMlhnd nnlmlNuln .nu ml HlVlhlUllHlHlsv mw .n llllllll lao 7' IME VARY/AIG VLMGE f- INVENToA Ana o/e M.' Gare Wl'C/l BY 20M 7- HS torng.

April 8, 1958 A. M. GUREwlTscH 2,830,222

APPARATUS FOR IMPARTING HIGH ENERGY TO CHARGED PARTICLES Original Filed Sept. 20, 1950 4 Sheets-Sheet 2 A111518, 1958 A. M. GUREwlTscH 2,830,222

APPARATUS FoR IMPARTING HIGH ENERGY To CHARGED PARTIGLRS original Filed sept.l 2o. 195o 4 sheets-sheet s Ilm II nlllll Il April 8, 1958 A. M. GuREwlTscH 2,830,222

APPARATUS FOR IMPARTING-HIGH ENERGY TO CHARGED PARTICLES Original Filed Sept. 20, 1950 4 Sheets-Sheet 4 Pfg. a.

United States Patent APPARATUS FOR IMPARTING HIGH ENERGY TO CHARGED PARTICLES Anatole M. Gurewitsch, Schenectady, N. Y., assignor to General Electric Company, `a corporation .of New York Griginal application September 20, 1954i, Serial No. 185,763', now Patent No. 2,673,928, dated March 30,

1954. Divided and this application February 10, 1954,

Serial No. 409,380

9 Claims. (Cl. WiSe-5.18)

possessed by certain orbits in apparatus such as that disclosed in U. S. Patent No. 1,948,384-Lawrence, commonly termed a cyclotron. As proposed, such a device involves the repeated application of an oscillating electric iield to charged particles as they move within a magnetic iield essentially normal to the electric tield at the point of application. In these circumstances charged particles, having an energy such that their angular velocity corresponds to the frequency of the electric field, will have an equilibrium energy and a stationary orbit if they arrive at the point of application of the oscillating electric ield asthe iield passes through zero. Particles which arrive at the point of application of the electric field either before or after the electric eld passes through zero will follow displaced orbits oscillating about the stationary orbit with both phase and energy varying about their equilibrium values. Acceleration of the particles may be accomplished by changing the value of the equilibrium energy, which, translated into practical terms, means varying either the magnetic iield orthe frequency of the electricl field.

Apparatus employing the above-mentioned phase stability principle at least in part has proved to be exceptionally successful in the acceleration of electrons to high energy levels. However, it has been considered inadvisable to utilize this principle throughout the acceleration cycle inasmuch as the electrons wouldI follow a relatively greatly expanding spiral orbituntil their velocity approaches that of light; consequently Vapparatus of this character has usually embodied other means for pre-accelerating electrons to approximately the velocity of light before initiation of means embodying the above-mentioned principle. lt is a foremost object of the present invention to provide simple, compact and emcient apparatus for accelerating charged particles to high energy levels by applying this phase stability principle of operation throughout the acceleration cycle after electron injection.

According to one important aspect of the present invention more fully explained and described hereinafter, there is provided a magnetic structure having a pair of opposed rotationally symmetrical pole pieces defining a gap between the pole faces thereof and means associated therewith for exciting the magnetic structure to produce a cyclic time-varying magnetic tieldfbetween the fpole faces. The pole faces are outwardly tapered to provide a desired varying radial YlieldY strength and an evacuated envelope is disposed within the gap dened therebetween.

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2. High frequency electric field directing means is positioned to enclose a portion of the evacuated envelope, and a source of charged particles, such as electrons, is suitably positioned for injection of .charged particles into the evacuated envelope. When the magnetic structure is excited, charged particles are injected at a desired time in the cycle of magnetic field and thereafter are accelerated in stable expanding spiral paths by the high frequency electric field directing means which acts repctitively to apply an accelerating force to the particles as they traverse lthe electric field generated thereby. When the charged particles have been accelerated to adesired energy level, they may be directed to a suitable target for the useful production of X-rays.

The features of the invention desired to be protected herein are pointed out in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in connection with the accompanying drawings in which Fig. 1 is a partially sectionalized View 4of accelerator apparatus suitably embodying the invention; Fig. 2 is a sectionalized view taken along line 2 2 of Fig. l with the discharge vessel partly broken away; Fig. 3 is a sectionalizedview taken along line 3--3 of Fig. 2; Fig. 3a is a fragmentary .View of the apparatus of Fig` l; Fig, 3b is a diagrammatic view useful in explaining the invention;l Fig. 4 is a simplified schematic representation of circuitry suitable for energizing the accelerator apparatus of the invention; Figs.k 5d and 5b are fragmentary views illustrating alternative means forexciting the resonant structure; Fig. 6 is a partially sectionalized View of one alternative form for the envelope and Aresonant structure of Fig. l; Fig. 7 is a partially sectionalized view taken 'along line '7 7 of Fig. 6; Fig. 8 is a view showing another alternative form of the envelope and'resonan't structure of Fig, 1 with the resonant structure partly broken away; and Fig. 9 is a sectionalized view taken along line 99` of Fig. 8.

Referring particularly to Figs. l, 2, and 3 there is shown a magnetic structure l which comprises yoke portions 2, leg portions 3 and-a pair of'opposed rotationally symmetrical pole pieces 4 and S, all of which may be laminated to prevent the generation of excessive eddy currentsV therein when structure l is excited by a timevarying magnetic eld. To provide a desired magnetic tield in the gap L6 between pole pieces 4 and 5, seriesconnected .energizing windings 7 and 8 are disposed respectively about pole pieces 4 and 5 and connected, as indicated, in circuit with power factor correcting capacitors 9 to a conventionally represented source of cyclic time-varying voltage 10. In order that the time-varying magnetic field existing in gap 6 when magnetic structure l is energized by windings 7 and 8 may have a desired radial variation in accordance with principles to be described hereinafter, pole pieces 4i and 5 are provided Arespectively `with pole faces l1i and 12 having an inverse slope with radius as shown.

Positioned within the gap-6 between pole faces 11 and 12 is a closed generally cylindrical vessel 13 of dielec.- tric material which defines within its interior a chamber 14. y As will be explained in greater detail at a later point, vessel or envelope 13 provides a spiral orbital path along which charged particles (i. e. electrons) may 'be accelerated to -high energylevels. Envelope 13 is preferably highly evacuated and is provided `with an axial side arm i5 of dielectric material which extends into a central opening 16 in magnetic structure l and supports a charged particle source 17 within chamber 14. Charged .par-

ticle source i7 maycornprise a shield member .18.and .a

lamentary cathode i9 supported ,endenergizedthrough cpnluctors ,.216 ,sealed .into Side erin 15j.V A morede.- tailed description of a'charged particle source suitable for employment in this connection `may be found in U. S. Patent 2,499,192, granted February 28, 1950, to James M. Latferty and assigned to the assignee of the present invention.

For the` purpose of providing means for accelerating electrons within, envelope 13 after they have been injected from charged particle source 17, there is shown high frequency field directing means 21 which encloses a portion of envelope 13 in gap 6 between pole faces 11 and 12. High frequency field directing means or resonator 21 comprises an inner conductor 22 which includes a box of dielectric material 23 supported upon the exterior surface of envelope 13. Box 23 may be openended or may be closed as shown at the end 24 remote from envelope 13. Attached to the outer sides of box 23 is a plurality of longitudinally extending spaced apart conductive strips 25 which may project over the end 24 of box 23 as is indicated at 26; Near end 27 opposite end 24 of box 23 conductive strips 25 may terminate adjacent the axis of envelope 13. To connect together or short-circuit strips 25, transverse peripheral conductive strip 28 is positioned adjacent the termination of strips 25 near end 27 of box 23. The outer conductor 29 of resonator 21 includes a rectangularshaped box of dielectric material 30 `upon the interior sides of which are positioned longitudinally extending spacedapart conductive strips 31 similar to those attached to inner box 23. Box 30 may also be open-ended, may have an inwardly projecting flange 32 to support the inward extensions 33 of strips 31, or may be completely closed at the end remote from envelope 13 as is indicated at 34. The inward extensions 33 of strips 31 are made in order that strips 31 may-be short-circuited to strips 25 to provide in effect a short-circuited quarter wave resonator as `will be more fully described hereinafter. Strips 31 are also short-circuited to each other by means of a transverse peripheral conductive strip 35 positioned opposite peripheral conductive strip 28.

Strips 25 and 31 may consist of the material such as copper and may be secured respectively to boxes 23 and 30 by means of a suitable adhesive.` material, i. e. an alkydresin prepared by reacting a polybasic acid and polyhydric alcohol, such as a resin being prepared from glycerol and phthalic anhydride. n To prevent excessive radiation losses when resonator 21 is excited by a source of high frequency, outer conductor 29 including the sides of box 30 and condductive strips 31 is extended beyond end 27 of 'inner conductor 22 as is shown at 36. Envelope 13 and inner conductor 22 may be supported from outer conductor 29 by means of an arcuate dielectric spacer 35', and `outer conductor 29 may be supported by arcuate dielectric spacer 36. Slots 37 and 37'l are provided in the undersides of outer conductor 29 and inner conductor 22 respectively to facilitate the insertion of envelope 13 within resonator 21. Means for conducting energy to resonator 21 may comprise a section of concentric transmission line 38 extending downwardly' through central opening 16 and being constructed of a cylinder 38' of dielectric material lined with a thin conductive strip 39 serving as an outer conductor.l Strip 39 extends around the lower end of dielectric cylinder 38 to make contact with a angedsleeve 40 positioned within an aperture 41 in box 30 and, in turn, making` contact with strips 31. Sleeve may be attached to liner 39 and strips 31 by any convenient means such as solder. The inner'conductor 42 of concentric transmission line section 38 may consist of a metallic rod 43 terminated with a disk 44 attached to peripheral conductive strip 28.

With the proper selection of longitudinal dimensions, v

such dimensions being in the direction indicated by the line labeled l in Fig. 2,V resonator 21 operates as Va short-circuited quarter wave concentric line resonator at a particular excitation frequency. The space between 4 conductive strips 25 and 31 constitutes in etect a space resonant system comprising a quarter wave transmission line section. Accordingly, if resonator 21 is excited at the proper frequency by a high frequency voltage source (not shown) connected to concentric line 38 in a manner well known to tho'se skilled in the art, a cyclically reversible electric field of high intensity may be made to appear between inner conductor 22 and outer conductor 29 as is indicated by arrows 45 of Fig. 3a. At the end of inner conductor 22 adjacent, the axis of envelope 13, however, the electric field will extend inwardly and within envelope 13 as is indicated by arrows 46. By choosing the frequency of reversal of this electric eld to correspond to the frequency of rotation of charged particles moving within envelope 13, i. e. equalto or a multiple of the frequency of rotation of the charged particles, an increase in the energy level of such charged Vparticles may be effected in accordance with the principles which will be presently described.

As has been mentioned heretofore, the present invention contemplates the initiation of excitation of resonator 21 early in the cycle of magnetic teld` between pole faces 11 and 12, i. e. at or shortly after the injection of electrons into envelope 13 from gun 17. After injection of electrons and the excitation of resonator 21 the electrons arerepetitively accelerated twice per revolution as they pass into and out of resonator 21. As is apparent from Fig. 3a, the region 47 within inner conductor 22 and envelope 13 is electric field-free; hence no accelera` tion occurs while-the electrons are Within this region. Thus the electrons are repetitively accelerated with a constant angular velocity throughout a number of revolu tions while they are retained within expanding spiral orbital paths by themagnetic field in gap 6 traversing envelope 13. In Fig. 3b wherein the ux density B in gap 6 is plotted vs. time (for present purposes B may be considered to be sinusoidal as shown), line a represents the `time at which electrons are injected from vgunj 17 and also the time at which resonator 21 is energized. The electrons are accelerated throughout the portion in; dicated by the shaded area under the B curve until. the time represented by line b is reached at or slightly after the peak magnetic ilux density is reached. At thistime resonator 21 may be de-energized whereby the electrons spiral outwardly to strike a target 48 (Figs. 1 and 2) of tungsten or other suitable material supported by a stud 49 sealed into the wall of envelope 13. i

In the construction of the above mentioned apparatus the following relations may be conveniently employed:

where R=the radius of the particle orbit l 11j-:the linear velocity of the particles, and w=the angular velocity of the particles Since in the above described apparatus w is maintained constant (by virtue of a constant frequency of excitation of resonator 21 and the above-described phase` stability principle) while v varies from the velocity of injection to, in some cases, nearly the velocity of light, the radius R will increase during acceleration of the particles to a value approaching the maximum value of where c=the velocity of light.

The total energy possessed by the particles, including rest energy and kineticenergy evidenced by a relativistic increase in mass, may be expressed as where B=the llux density of the magnetic field traversing the particle orbit. t

It may now be shown that mr-the rest mass of the particles, and e=the charge upon the particles where If now B is made to vary with time,ri. e. sinusoidally as represented in Fig. 3b, then at the time of injection ta, B will be determined by the relation Ba=B0 sin wml.,

where wm=21rf the frequency of the magnetic field, and B0=the peak value of magnetic flux density ing necessary quantity which must be determined, viz.

the necessary gain of energy of the particles per turn, may be determined from the following equation:

where f=the frequency of the electric field produced by resonator 21, andv fm=the frequency of the magnetic ield (e. g. 60 cycles) At higher energies where radiation loss takes place, an additional amount of energy per turn must be supplied as is indicated by the following relation:

E4 L=8.8 T l032 (7) where L=the gain per turn in electron volts necessary to compensate for radiation losses.

From the above stated relationships' it may be ascertained that, for the acceleration of electrons tov a final energy of 100 million electron volts, a ux density of .8 weber/meterz and a final orbit radius of approximately 42 centimeters may be employed. The frequency of the electric eld produced by resonator 21 may be of 'the order of 160 megacycles.

In order to maintain stability of the charged particles undergoingy acceleration to produce a desired stablebeam, the magnetic flux density B in the plane of the orbit in gap 6 must satisfy the following relation:

r=the radius of the point under consideration r0=the maximum radius of the orbit, and t n=an exponential having a value lying between 0 and unity Otherwise expressed thisrelation isi where the above-described apparatus. A pulse generator 50 supplies intermittent energization through conductors 20 to electron gun 17 (not shown) at a desired instant in the cycle of magnetic eld produced by energizing Windings 7 and 8 as determined by a saturable strip 51 around which is positioneda winding 52. It will be understood by those well skilled in the art that saturable strip 51 may be arranged such that it will produce a triggering pulse in winding 52 at a desired time instant in the cycle of magnetic field and that this pulse may be directed through conductors 53 to pulse generator 50 for initiating the energi'zation thereof. In a similar manner, high frequency voltage source 54 may be triggered by means of a saturable strip 55 coupled with a winding 56 which is connected through conductors 57 to high frequency voltage source 54. For a more complete description of circuitry which may be suitably adapted for these purposes, reference may be had to U. S, Patent 2,485,409, issued October 18, 1949, to Herbert C. Pollock and Willem F.V Westendorp and assigned to the assignee of the present invention.

In the fragmentary elevation of Fig. 5a, wherein similar numerals are used to designate like parts hereinbefore described, there is shown alternative means for exciting resonator 21. ln this embodiment resonator 21 is excited through a concentric transmission line 58 suitably introduced through the side of dielectric box 36 near the end remote from envelope 13. Proper magnetic coupling of a high frequency voltage source (not shown) is assured by making a right angle loop in the inner conductor 59 as is illustrated in the fragmentary view of Fig. 5b'. Loop portion 60 of inner conductor 59 should be perpendicular to the plane of the paper in Fig'. 5a and may be directed either into or out of the plane of the paper.

Referring now to Figs. 6 and 7 there is shown an alter` native resonator and envelope structure which maybe employed in connection with the apparatus illustrated in Fig. 1. In this embodiment a generally cylindrical envelope 113 is supported by means of an arcuate spacer member 135 within a resonator 121. Resonator 121 comprises an inner conductor 122 which includes spaced apart radial conductive strips 125 attached to the outer surface of envelope 113 and terminating at their in ner ends in a semi-circular shorting strip 128. Outer conductor 129 comprises a generally cylindrical box portion 130 of dielectric material enclosing a portion of envelope 113 and having attached to its inner'surface a plurality of spaced apart radial lconductive strips 131 which terminate at their inner ends on a conductive short-'circuiting strip 135. To facilitate the insertion of envelope 113 within outer conductor 139 the latter is provided with a trans"- verse slot 137 as shown. High frequency energy may be introduced into the structure by means of a concentric transmission line portion 138 which is directly coupled into a concentric line section or stub 138' as illustrated. If( more convenient, portion 138 may be coupled into the outer end of stub 12:8 as will be understood by those skilled in the art. Stub 138 comprises a cylindrical portion 139' of dielectric material which is lined with thin conductive material 139. At its lower end the liner 139 is connected to strips 131 as illustrated at 140, and at its upper end it is short-circuited to an inner conductor 142 whichV comprises a metallic rod 143 extending downwardly and terminating in a diskV 144 which makes contact with strips 125. With the proper selection ofk dimensions this structure also operates as a quarter wave concentric line resonator at a particular excitation frequency. The space between conductive strips 125 on the exterior surface or" envelope 113 and conductive strips 131 on the interior surface of dielectric box may be considered as" forming a lumped capacitance across the inner end of concentric line stub or section 138. Accordingly, if the structure has the proper dimensions and is excited at the proper frequency, a cyclically reversibleA electric field of high intensity may be made to appearl across the inner and outer conductors in a manner similar to -that described in connection with Fig. 3a.

In Figs. 8 and 9 there is shown another alternative form of resonator and envelope structure which may be advan! tageously employed in connection with the apparatus of Fig. l. In this embodiment charged particles may be accelerated within a closed annular envelope 200 of dielectric material within the Wall of which there are sealed energizing conductors 201 supporting a charged particle source or electron gun 202.v Enclosing a portion of an nular envelope 200, there is shown resonator structure which comprises an inner conductor 203 including a plurality of spaced coaxial conductive strips 204 suitably attached to the outer surface of envelope 200. An outer conductor 205 comprises a plurality of spaced coaxial conductive strips 206 suitably attached to the interior surface of an annular section 207 of dielectric material. Loops 203 along with kenvelope200 may be supported in spaced coaxial relationship with respect to outer conduc- 'i tor 205 by means of dielectric spacers 208. Loops 204 and 206 have discontinuous peripheries to form respectively aligned gaps 209 and 210. Positioned along the outer surface of annular section 207 are three arcuate spaced parallel plates `211, 212, and 213 forming extensions of inner conductor 203 and outer conductor 205. The inner conductive plateprojects through gap 210 and terminates at its inner end in gap 209 to make contact with loops 204 as shown. The two outer plates211` and 213 terminate in gap 210 to make contact with loops 206. At their outer or distal ends plates 211,212, and 213 are short-circuited by an arcuate plate 214. Highfrequency energy may be conducted into the resonator structure by means of a concentric line 215 having an inner .Y

conductor 216 connected directly to plate 212v and an outer conductor 217 connected `directly to plate 213. 1t will be now understood by those skilled in the art that this structure will operate in a similar fashion `to that disclosed in Figs. 6 and 7 to accelerate charged particles twice per revolution as they pass into and out of the structure. The space between conductive loops 204 and 206 may be considered as forming a lumpedy capacitance across the inner end of parallel plates 211, 212, and 213 which act as a transmission line short-circuited at the opposite end.

From the foregoing description of the various embodiments of the inventionit will be apparent that advantagcous accelerator apparatus is provided. Since it is not necessary to produce a time-varying magnetic flux linking the charged particle orbit to pre-accelerate the particles,

opening 16 may be employed as ka means of reducing the amount of iron in magnetic structure 1 and also as a means of introducing various lead-in conductors. Furthermore, `the strips from which the resonator structure are constructed minimize the generation of eddy-currents by the time-varying magnetic ticld. Moreover, large radial spaces are available for initial oscillations of the charged particles about the orbit and, therefore, the increase in output of the apparatus is facilitated.

What I claim as new and desire to secure by Letters Patent of the United States is: t

l. ln combinatioma closed annular envelope of dielectric material within which charged particles may` be accelerated, means providing a source of charged particles within the envelope and a high frequency resonatorenclosingy a portion of said envelope, said resonator com pricing an outer conductor which includes a hollow annular section of dielectric material having radially ex- Sli tending conductive loops spaced along its inner surface and a circumferentialgap along its outer periphery, an inner conductor which includes radially extending conductive loops spaced along the outer surface of said envelope, and spaced parallel plates forming extensions of said outer and inner conductors through said circumferential gap in said outer conductor.

2. In combination, a closed annular envelope of dielectric `material within which charged particles may be ac celerated, means providing a source of charged particles within the envelope and a high frequency resonator enclosing a portion of said envelope, said resonator cornprising an outer conductor which includes a hollow annular 'section of dielectric `material having spaced radially extending conductive loops along its inner surface and a circumferential gap 4along its outer periphery, an inner conductorwhich includes radially extending conductive loops spaced along the, outer surface of said envelope, and spaced parallel plates forming extensions ofsaid inner and outer conductors in planes perpendicular to the axis of said envelope through said circumferential gap in said outer conductor.

3. In combination, 'a closed annular envelope of dielectric material within which charged particles may be accelerated, means providing a source of charged particles within the envelope and a high frequency resonator enclosing a portion ofl said envelope, said resonator comprising an outer conductor which includes a hollow annular section of dielectric material having spaced radially extending conductive loops along its inner surface and a circumferential gap along its outer periphery, an inner conductor which includes radially extending conductive loops spaced along the outer surface of said envelope, and spaced parallel` plates forming extensions of said inner and outer conductors in planes perpendicular to the axis of said envelope through said circumferential gap in said outer conductor, said spaced parallel plates being terminated attheir ends remote from said inner and outer conductors by a short-circuiting plate.

4. In combination, a closed annular envelope of dielectric material within which charged particles may be ac-` celerated, means yproviding a source of charged particles within the envelope a highfrequency resonatorenclosing a portion of said envelope, said resonator comprising an outer conductor which includes a hollow annular section of dielectric material having spaced radially extending conductive loops along its innersurface and a circumferential gap along its outer periphery, an inner conductor which includes radially extending conductive loops spaced l along the outer surface of said envelope, and spaced parallel plates forming extensions of said inner and outer conductors in planes perpendicular to the axis of `said envelopethrough said circumferential gap in said outer conductor, said spaced parallel plates being short-circuited at their distal ends, and means for exciting said resonator comprising a section of coaxial transmission line coupled to said parallel plates.

5. A resonatorstructure comprising an inner conductor which includes la plurality of spaced coaxial conductive loops, an outer conductor which includes va plurality of spaced coaxial conduetiveloops, and three spaced parallel plates extending essentially perpendicular to the common axis of said inner and outer conductors, the two outer plates being attached to said outer conductor and the central plate being attached to said inner conductor.

6. A resonator structure comprising an inner conductor which includes a plurality of spaced coaxial conductive loops, an outer conductor which includes a plurality of spaced coaxial conductive loops, `and three spaced parallel plates extending essentially perpendicular to the common axis of said inner and outer conductors, the two outer plates 'being attached to'said outer conductor and the central plate being attached to said inner conductor, all said .three plates being interconnected at their distal ends.

7. A resonator structure comprising an inner conductor which includes a plurality of spaced coaxial conductive loops, an outer conductor which includes a plurality of spaced conductive loops having aligned gaps in the peripheries thereof, and three spaced parallel plates forming extensionsv of saidinner `and outer conductors, the two outer plates being attached to said outer conductive loops within the gaps therein, the central plate extending through said gaps in said outer conductive loops and being attached to said inner conductive loops.

S. A resonator structure comprising an inner conductor Which includes a plurality of spaced coaxial conductive loops having aligned gaps in vthe peripheries thereof, an outer conductor which includes a plurality of spaced Coaxial conductive loops having aligned gaps in the peripheries thereof, the gaps in said inner and outer conductors being transversely aligned, and three spaced parallel plates forming extensions of said inner and outer conductors, the two outer plates being attached to said outer conductive loops Within the gaps therein and the central plate being attached to said inner conductive loops within the gaps therein.

9. A resonator structure comprising an inner conductor which includes a plurality of spaced coaxial conductive loops having aligned gaps in the peripheries thereof, an outer conductor which includes a plurality of spaced coaxial conductive loops having aligned gaps in the peripheries thereof, the gaps in said inner and outer conductors being transversely aligned, and three spaced parallel plates forming extensions of said inner and outer conductors, the two outer plates being attached to said outer conductive loops Within the gaps therein and the central plate being attached to said inner conductive loops within the gaps therein, the said three spaced parallel plates being interconnected at their distal ends.

References Cited in the le of this patent UNITED STATES PATENTS 2,402,948 Carlson July 2, 1926 2,500,749 Gurewitsch Mar. 14, 1950 2,523,725 Schmidt Sept. 26, 1950 2,579,315 Gurewitsch Dec. 18, 1951 2,593,095 Brehm Apr. 15, 1952 2,641,734 Sloan June 9, 1953 2,673,928 Gurewitsch Mar. 30, 1954 

